I'd be very curious how a WP plugin is managing this.
Third party hosting building in caching, normal in-line caches (like Varnish), nginx/apache caching, sure - I get those, and they behave how you say. But a WP plugin? I'm curious how that would bypass PHP & WP entirely.
On the other hand, this trick was mostly useful years ago when shared hosting plans were really underpowered.
As of today, with improvements of both PHP 7.x and general server performance, even a cache system that uses PHP very early (before any DB connection) to load a pre-rendered html cache file goes a long way in terms of number of pages being served.
Actually, if this become your bottleneck, the .htaccess trick won't help a lot, and you would still look in tools like memcached, redis or reverse proxies to improve things.
I think a lot of third-party hosts provide this out of the box, maybe even using their own CDN. WP is so popular that it's be difficult not to find a service that handled it all for you so long as you have room in the budget.
If you don't know how the cache plugins do this, it's pretty obvious you are not very knowledgable about WordPress site operations. Please stop spouting falsehoods here.
I'm not a big WP fan, but this statement is false in most cases. Page caching in WordPress is usually (always?) serving up the pre-rendered HTML direct from disk (or CDN) without hitting PHP. Pagely (the post we're discussing) even handle that for you w/ their own CDN. I would imagine they also support request based caching and purging, warming, etc.
Most purely content sites (non-app) can run really well with a very simple TTL based cache (instead of long lived w/ complex flushing). You can very easily use something like Fastly for cloud based varnish w/ an appropriate TTL.
I also feel the need to point out that gatsby type sites and just pre-rendering / caching as well, at least in part (possible using json from a CMS for dynamic data).